CN106351731B

CN106351731B - Method and system for boost control

Info

Publication number: CN106351731B
Application number: CN201610554574.2A
Authority: CN
Inventors: 肖柏韬; H-R·欧萨乐; J·E·M·赫尔斯特伦; M·A·桑蒂洛
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2015-07-14
Filing date: 2016-07-14
Publication date: 2020-08-07
Anticipated expiration: 2036-07-14
Also published as: DE102016111692B4; DE102016111692A1; US9695740B2; US20170016389A1; CN106351731A

Abstract

The invention relates to a method and a system for boost control. Methods and systems are provided for rotational speed control of a turbocharger in an engine system having a plurality of staged charging devices. In one example, compressed air is provided through a turbocharger compressor until turbocharger speed reaches a limit. Thereafter, the compressors of the downstream superchargers are operated to share the load.

Description

Method and system for boost control

Technical Field

The present description relates generally to methods and systems for enabling pressure control in a staged boosted engine system configured with a turbocharger and a supercharger.

Background

The engine system may be configured with a boosting device (such as a turbocharger or supercharger) for providing a charge of charge air and improving peak power output. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with the added benefit of fuel economy. Further, one or more intake air charging devices may be staged in series or parallel with the intake turbocharger to improve the boost response of the turbocharged engine.

One example of a multi-stage intake charging system is shown by Stewart in U.S. patent 7,958,730. Wherein the high pressure turbine is staged upstream of the low pressure turbine, each turbine being coupled to a corresponding compressor. The multi-stage configuration allows multiple degrees of freedom in a boosted engine system, enabling control of two set points, one of which includes boost pressure.

However, the inventors herein have identified potential problems with using such multi-stage systems. As one example, turbocharged engine systems may have several hardware limits, such as maximum turbocharger shaft speed, which may be violated when operating the vehicle at high engine loads or at altitude. Likewise, the rotational speed limit of the turbocharger cannot be exceeded due to the potential for substantially immediate mechanical degradation. Current control systems may address this problem by reducing the maximum boost pressure when such constraint violations are anticipated. Further, the airflow actuator may be adjusted to reduce boost pressure, such as by opening an exhaust wastegate and/or a compressor recirculation valve. However, a drop in boost output to the driver-demanded boost pressure may result in a significant under-delivery of the torque request, and a drop in vehicle drivability. In addition, the driving experience of the vehicle operator is reduced.

Disclosure of Invention

In view of these problems, a method for improving rotational speed control of a turbocharger component in a boosted engine having a plurality of staged charging devices is provided. The method comprises the following steps: bypassing the second compressor and providing a flow of compressed air to the piston engine via the first compressor; and accelerating the second compressor in response to the shaft speed of the first compressor being equal to or above a threshold. In this way, turbocharger speed control is enabled without degrading supercharged engine performance.

As one example, a boosted engine system may include an electro-mechanical supercharger coupled upstream of a turbocharger. For example, a supercharger may be coupled upstream of the charge air cooler. An electromechanical supercharger may be used to provide compressed air to the engine during conditions when boost is required and at the same time turbo spin up (turbo up). Then, once the turbine spins up, the turbocharger compressor can be used to provide compressed air to the engine while bypassing the supercharger. Turbocharger shaft speed may be monitored based on, for example, turbine speed or turbocharger compressor speed. If the turbocharger compressor shaft reaches a speed limit (or if the turbocharger compressor or turbine speed reaches a speed limit) while the desired boost pressure is provided by the turbocharger, the electric supercharger may spin up to reduce the load on the turbocharger. Specifically, the supercharger may be activated to share a portion of the total demand power. The operating speed of the turbocharger is reduced due to load sharing between the turbocharger and the supercharger. The supercharger compressor may be rotated via one or more of the electric motor and the engine crankshaft at a rotational speed based on the turbocharger shaft speed, which increases when the turbocharger shaft speed exceeds a rotational speed limit. Additionally, the ratio of power delivered to the supercharger via the electric motor relative to the crankshaft may also be adjusted based on the turbocharger shaft speed. Once the turbocharger speed has been controlled, the supercharger may be disabled and compressed air may again be provided via the turbocharger.

A technical effect of sharing the boost load of the first downstream turbocharger compressor via operation of the second upstream supercharger compressor is that turbocharger compressor overspeed can be addressed without degrading supercharged engine performance. By operating the supercharger to reduce the boost load provided by the turbocharger, the speed of the first compressor can be reduced without the need to reduce boost pressure at the first compressor via operation of the compressor recirculation valve or wastegate. By reducing turbocharger speed, component life is extended. By using a supercharger to both reduce turbocharger speed and maintain boost pressure, insufficient delivery of torque requests is avoided and vehicle drivability is not degraded. In general, the performance of a boosted engine system with a staged charge device is improved.

It should be understood that the summary above is provided to introduce in simplified form a concept that is further described in the detailed description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.

Drawings

FIG. 1 illustrates an example embodiment of a boosted engine system with multiple staged charge boosting devices.

FIG. 2 shows a high level flow chart illustrating a routine that may be implemented for reducing the rotational speed of a turbocharger compressor by operating the supercharger compressor.

FIG. 3 shows a high level flow chart illustrating a routine that may be implemented for accelerating a supercharger compressor using a variable ratio of power from an electric motor to power from an engine crankshaft.

FIG. 4 illustrates an example adjustment that may be used to reduce overspeed of a turbocharger staged downstream of a supercharger compressor.

Detailed Description

The following description relates to systems and methods for improving turbocharger speed control in an engine system having a staged boosting device, such as in the boosted engine system of fig. 1. The controller may be configured to execute a routine (such as the example routines of fig. 2-3) to increase the speed of the downstream compressor to reduce the load, and to decrease the temperature of the upstream compressor. An example temperature control operation is illustrated with reference to fig. 4. By operating the second compressor, the over-temperature of the first compressor can be suppressed while continuing to meet the driver torque demand.

FIG. 1 schematically illustrates aspects of an example engine system 100 including an engine 10. In the depicted embodiment, engine 10 is a supercharged engine including a plurality of staged superchargers. Specifically, the engine 10 includes a first supercharging device 13 that is staged downstream of a second supercharging device 15. This configuration results in the first compressor 114 (of the first boosting device) being positioned in the engine intake 42 downstream of the second compressor 110 (of the second boosting device). In the present example, the first supercharging device is a turbocharger 13 and the second supercharging device is an electromechanical supercharger 15.

The turbocharger 13 includes a first compressor 114 driven by a turbine 116. First compressor 114 is shown as a turbocharger compressor mechanically coupled to turbine 116 via shaft 19, turbine 116 being driven by expanding engine exhaust gases. In one embodiment, the turbocharger may be a twin scroll device. In another embodiment, the turbocharger may be a Variable Geometry Turbocharger (VGT), wherein the turbine geometry is actively varied depending on engine operating conditions. Fresh air is introduced into engine 10 along intake 42 via air cleaner 112 and flows to second compressor 110. During selected conditions, as described in detail below, air compressed by the turbocharger 13 may be recirculated from the outlet of the compressor 114 to the inlet of the compressor 114 through the first compressor bypass 60 by adjusting the opening of the Compressor Recirculation Valve (CRV) 62. The CRV 62 may be a continuously variable valve, and increasing the opening of the recirculation valve may include actuating (or energizing) a solenoid of the valve.

As shown in FIG. 1, first compressor 114 is coupled to throttle 20 through a Charge Air Cooler (CAC)18 (also referred to herein as an intercooler). The throttle 20 is coupled to an engine intake manifold 22. Air is received at the first compressor from the second compressor 110. The compressed air charge from the first compressor flows through the charge air cooler 18 and the throttle valve to the intake manifold. For example, the charge air cooler may be an air-to-air or water-to-air heat exchanger. In the embodiment shown in FIG. 1, the pressure of the air charge within the intake manifold is sensed by a Manifold Air Pressure (MAP) sensor 124.

The electro-mechanical supercharger 15 includes a second compressor 110 driven by an electric motor 108. Specifically, the compressor fan is driven by power received from the electric motor along the supercharger compressor shaft 80. The motor 108 is powered by an onboard energy storage device, such as the system battery 106. The air compressed by the second compressor 110 is then delivered to the first compressor 114. Fresh air received at the compressor inlet of the first compressor 114 is then introduced into the engine 10. During selected conditions, as described in detail below, air may bypass the supercharger 15 and be directed through the second compressor bypass 70 by adjusting the opening of the bypass valve 72. During these conditions, compressed air may be delivered to engine 10 via only turbocharger first compressor 114.

Additionally, the second compressor 110 of the supercharger 15 may be driven by the engine crankshaft via a clutch and gear mechanism. Likewise, engine torque may be transferred to the vehicle wheels 47 via the driveline shaft 84. Specifically, engine torque may be relayed from crankshaft 40 to transmission 48, and thereon to wheels 47. Transmission 48 may be a fixed ratio transmission that includes a plurality of gear ratios to allow engine 10 to rotate at a different speed than wheels 47. A clutch (not shown) may be disposed between the engine crankshaft 40 and the transmission 48. By varying the torque transfer capacity of the clutch (e.g., the amount of clutch slip), the amount of engine torque relayed to the wheels via the driveline shaft may be adjusted. In addition to transferring engine torque to the vehicle wheels, engine torque may be transferred to the supercharger compressor shaft 80 via a driveline 86. Specifically, engine torque may be relayed from crankshaft 40 to supercharger compressor shaft 80 at a location downstream of electric motor 108 via clutch 82. By varying the torque transfer capacity of the clutch 82 (e.g., the amount of clutch slip), the amount of engine torque (relative to motor torque) used to drive the supercharger compressor may be adjusted. Although not shown, a plurality of gear ratios may be included in the drivetrain 86 to allow the engine 10 to rotate at a different speed than the supercharger compressor 110. It will be appreciated that in alternative embodiments, the drivetrain 86 may be coupled to the supercharger compressor shaft 80 at a location upstream of the electric motor 108. The ratio of power that electrically drives the supercharger compressor via the electric motor relative to power that mechanically drives the supercharger compressor via the engine crankshaft may be adjusted based on engine operating conditions, as detailed in fig. 3.

It is to be understood that as used herein, the first compressor refers to downstream of the staged compressor and the second compressor refers to upstream of the staged compressor. In one non-limiting example, as depicted, the first downstream compressor is a turbocharger compressor and the second upstream compressor is a supercharger compressor. However, other combinations and configurations of the boosting devices are possible.

Turbo lag can occur when operating from an engine that does not require boosting to an engine that requires boosting during selected conditions, such as during tip-in. This is due to the delay in turbine spin-up and the reduced flow through the first compressor 114 when the throttle is opened when the accelerator pedal is depressed. To reduce the turbo lag, both the supercharger 15 and the turbocharger 13 may be activated during those selected conditions. Specifically, with turbine 116 spinning up, boost pressure can be provided by downstream supercharger compressor 110. Activating the supercharger includes drawing energy from the battery 106 to rotate the motor 108, thereby accelerating the second compressor 110. Additionally, the bypass valve 72 may be closed to enable a larger portion of the air to be compressed by the second compressor 110. Then, when the turbine has spun sufficiently fast and is able to drive the first compressor 114, the second compressor may be slowed by disabling the motor 108, and additionally, the bypass valve 72 may be opened to enable a larger portion of air to bypass the second compressor 110.

Likewise, a turbocharger may have several hardware limits, such as a maximum turbocharger shaft speed. If the turbocharger shaft speed exceeds this maximum value for a certain period of time, turbocharger stability may degrade due to an immediate mechanical failure. As detailed herein in fig. 2-3, the controller may reduce the load on the turbocharger compressor by activating the turbocharger compressor during conditions when a constraint violation is detected or predicted, such as when it is predicted or detected that the turbocharger shaft speed (inferred based on turbine speed or turbocharger compressor speed) will be above a threshold. Specifically, the second compressor may be reactivated and accelerated, resulting in a drop in load at the first compressor. By operating the second compressor to reduce the load on the first compressor, the turbocharger compressor speed of the first compressor can be reduced without requiring engine airflow to be curtailed and without requiring insufficient engine torque to be delivered.

Compressor surge can occur during selected conditions, such as during accelerator pedal release, when operating from an engine with boost to an engine without boost or reduced boost. This is due to the reduced flow through the first compressor when the throttle is closed when the accelerator pedal is released. Reduced forward flow through the first compressor can cause surge and degrade turbocharger performance. In addition, surge can cause NVH problems, such as undesirable noise from the engine intake system. To reduce compressor surge, at least a portion of the intake air compressed by the first compressor 114 may be recirculated to the compressor inlet. This allows the excess boost pressure to be substantially instantaneously reduced. The compressor recirculation system may include a recirculation passage 60 including a compressor recirculation valve 62 for recirculating (warm) compressed air from a compressor outlet of the first compressor 114 to a compressor inlet of the first compressor 114 upstream of the charge air cooler 18. In some embodiments, the compressor recirculation system may alternatively or additionally comprise a recirculation channel for recirculating (cooled) compressed air from the compressor outlet to the compressor inlet downstream of the charge air cooler.

One or both of valves 62 and 72 may be continuously variable valves, wherein the position of the valve is continuously variable from a fully closed position to a fully open position. Alternatively, compressor recirculation valve 62 may be a continuously variable valve, while compressor bypass valve 72 is an on-off valve. In some embodiments, the CRV 62 may be normally partially opened during supercharged engine operation to provide some surge margin. Herein, the partially open position may be a default valve position. Then, in response to the indication of surge, the opening of the CRV 62 may be increased. For example, the valve(s) may be moved from a default partially open position toward a fully open position. During those conditions, the degree of opening of the valve(s) can be based on an indication of surge (e.g., compressor ratio, compressor flow rate, pressure differential across the compressor, etc.). In an alternative example, the CRV 62 may remain closed during boosted engine operation (e.g., peak performance conditions) to improve boost response and peak performance.

One or more sensors may be coupled to an inlet of the first compressor 114 (as shown) and/or the second compressor 110 (not shown). For example, a temperature sensor 55 may be coupled to the inlet for estimating the compressor inlet temperature. As another example, a pressure sensor 56 may be coupled to the inlet for estimating the pressure of the intake air entering the compressor. However, other sensors may include, for example, an air-fuel ratio sensor, a humidity sensor, and the like. In other examples, one or more compressor inlet conditions (such as humidity, temperature, etc.) may be inferred based on engine operating conditions. The sensors may estimate conditions of intake air received from the intake at the compressor inlet and recirculated intake air upstream from the CAC. One or more sensors may also be coupled to intake passage 42 upstream of

compressors

114 and 110 for determining the composition and condition of intake air entering the compressors. These sensors may include, for example, a manifold airflow sensor 57.

Intake manifold 22 is coupled to a series of combustion chambers 30 via a series of intake valves (not shown). The combustion chambers are further coupled to an exhaust manifold 36 via a series of exhaust valves (not shown). In the depicted embodiment, a single exhaust manifold 36 is shown. However, in other embodiments, the exhaust manifold may include a plurality of exhaust manifold portions. Configurations having multiple exhaust manifold portions may enable effluents from different combustion chambers to be directed to different locations in an engine system.

In one embodiment, each of the exhaust and intake valves may be electronically actuated or controlled. In another embodiment, each of the exhaust and intake valves may be cam actuated or controlled. Whether electronically or cam actuated, the timing of the opening and closing of the exhaust and intake valves may be adjusted as needed for desired combustion and emission control performance.

Combustor 30 may be supplied with one or more fuels such as gasoline, alcohol fuel blends, diesel, biodiesel, compressed natural gas, and the like. Fuel may be supplied to the combustion chamber via direct injection, port injection, throttle body injection, or any combination thereof. In the combustion chamber, combustion may be initiated via spark ignition and/or compression ignition.

As shown in FIG. 1, exhaust gas from one or more exhaust manifold sections is directed to a turbine 116 to drive the turbine. When reduced turbine torque is desired, some exhaust gas may instead be directed through the wastegate 90, bypassing the turbine. A wastegate actuator 92 may be actuated to open to dump at least some exhaust gas pressure from upstream of the turbine to a location downstream of the turbine via a wastegate 90. By reducing the exhaust pressure upstream of the turbine, the turbine speed can be reduced.

The combined flow from the turbine and the wastegate then flows through the emissions control device 170. Generally, one or more emission control devices 170 may include one or more exhaust aftertreatment catalysts configured to catalytically treat the exhaust flow and thereby reduce the amount of one or more substances in the exhaust flow. For example, an exhaust aftertreatment catalyst may be configured to trap NO from an exhaust gas flow when the exhaust gas flow is lean_xAnd reducing trapped NO when the exhaust stream is rich_x. In other examples, the exhaust aftertreatment catalyst may be configured to disproportionate (disproportionate) NO_xOr selective reduction of NO by means of a reducing agent_x. In other examples, the exhaust aftertreatment catalyst may be configured to oxidize residual hydrocarbons and/or carbon monoxide in the exhaust stream. Different exhaust gas aftertreatment catalysts having any such function may be arranged individually or together in the washcoat or elsewhere in the exhaust gas aftertreatment stage. In some embodiments, the exhaust aftertreatment stage may include a regenerable soot filter configured to trap and oxidize soot particulates in the exhaust flow.

All or a portion of the treated exhaust from emission control 170 may be released to the atmosphere via exhaust conduit 35. However, depending on operating conditions, some exhaust gas may instead be diverted to the intake port via an EGR passage (not shown) that includes an EGR cooler and an EGR valve. The EGR may be recirculated to the inlet of the first compressor 114, the second compressor 110, or both.

The engine system 100 may also include a control system 14. The control system 14 is shown receiving information from a plurality of sensors 16 (various examples of sensors are described herein) and sending control signals to a plurality of actuators 81 (various examples of actuators are described herein). As one example, the sensors 16 may include an exhaust gas sensor 126, a MAP sensor 124, an exhaust gas temperature sensor 128, an exhaust gas pressure sensor 129, a compressor inlet temperature sensor 55, a compressor inlet pressure sensor 56, and a MAF sensor 57 located upstream of the emissions control device. Other sensors such as additional pressure, temperature, air/fuel ratio, and constituent sensors may be coupled to different locations in the engine system 100. Actuators 81 may include, for example, throttle 20, compressor recirculation valve 62, compressor bypass valve 72, electric motor 108, wastegate actuator 92, and fuel injector 66. The control system 14 may include a controller 12. The controller may receive input data from various sensors, process the input data, and employ various actuators based on the received signals and instructions stored on a memory of the controller. Based on instructions or code programmed therein corresponding to one or more programs (such as the example control programs described herein with reference to fig. 2-3), the controller may employ the actuators in response to processed input data.

Turning now to fig. 2, an example routine 200 is shown for operating a compressor of a downstream boosting device (e.g., a turbocharger) in response to temperature constraints of the compressor of the downstream boosting device (e.g., a turbocharger). Based on instructions stored on a memory of the controller in combination with signals received from sensors of the engine system (such as the sensors described above with reference to fig. 1), the instructions for implementing the method 200 and the remaining methods included herein may be executed by the controller. The controller may employ engine actuators of the engine system to adjust engine operation according to the methods described below.

At 202, the method includes estimating engine operating conditions, such as engine speed, pedal position, operator torque request, ambient conditions (ambient temperature, pressure, humidity), engine temperature, and the like. At 204, the method includes determining whether pressurization is required. In one example, boost may be required at medium to high engine loads. In another example, boost may be required in response to an increase in operator pedal tip-in or driver torque demand. If boost is not desired, such as when engine load is low or driver torque demand is low, then the method moves to 206 where the engine is operating with natural aspiration.

If boost is desired, at 208, the method includes starting the second upstream compressor while the turbine coupled to the first downstream compressor spins up. Herein, the second compressor is accelerated and increased to the compressed air flow of the engine in response to an increase in driver demand torque. Herein, the second compressor is staged upstream of the first compressor along the air intake. Further, the second compressor is driven by an electric motor, while the first compressor is driven by an exhaust turbine. In one example, as shown with reference to fig. 1, the first compressor is a turbocharger compressor and the second compressor is a supercharger compressor. Herein, accelerating the second compressor includes rotating the second compressor via the electric motor using power drawn from the battery. For example, the second compressor may be rotated by adjusting an electromechanical actuator coupled to an electric motor of the supercharger to rotate the motor at a higher rotational speed by sending a control signal from the controller to the actuator. The second compressor is accelerated at an increased rotational speed based on the boost demand. Thus, compressed air is provided to the engine via the second compressor.

Likewise, an electromechanical supercharger may have a response time of 130ms-200ms (i.e., idling to 100% duty cycle), and thus may be able to deliver boost much faster than the response time of a typical turbocharger (1 second-2 seconds). Thus, the second compressor of the electro-mechanical supercharger may be able to fill the turbo lag significantly faster.

When exhaust heat and pressure are generated due to in-cylinder combustion, the exhaust turbine speed increases, thereby driving the first compressor. At 210, it is determined whether the turbine speed is above a threshold, such as above a threshold where the turbocharger can maintain boost requirements. If not, then at 212, operation of the second compressor (of the supercharger) is maintained.

If the turbine speed is above the threshold, then at 214, the method includes decelerating the second compressor by disabling the electric motor, for example, based on a signal from the controller to an electromechanical actuator of the motor to reduce the motor speed. Additionally, a bypass valve (such as bypass valve 72) may be opened, allowing air compressed by the first compressor to bypass the second compressor and flow to the engine. Specifically, by sending a control signal from the controller to the actuator, an electromechanical actuator coupled to the bypass valve in the bypass across the second compressor may be adjusted to rotate the bypass valve to a more open position. Thus, after the turbine has spun sufficiently, the method includes bypassing the second compressor and providing a flow of compressed air to the piston engine via the first compressor. Here, the compressed air is not provided to the engine via the second compressor. In this way, by momentarily operating the second compressor of the supercharger until the turbocharger turbine spins up, turbo lag due to the delay in spinning up the first compressor is reduced.

At 216, it may be determined whether the shaft coupled to the first compressor is operating at or near a hardware limit, such as a shaft speed limit. Specifically, it may be determined whether a rotational speed of a shaft coupled to the first compressor shaft is equal to or above a threshold rotational speed. In one example, it may be determined whether the first compressor shaft speed is above 191,000 RPM. Likewise, since the turbocharger shaft couples the first compressor to the turbine, in an alternative example, it may be determined whether the first compressor speed is equal to or greater than a threshold speed, or whether the turbine speed is equal to or greater than a threshold speed. In one example, the turbine speed evaluated at 210 may be compared against a first, lower threshold speed to determine whether to deactivate the second compressor. In contrast, the turbine speed evaluated at 216 may be compared against a second, higher threshold speed to determine whether the hardware limit has been reached and whether to reactivate the second compressor.

If the turbocharger shaft speed (as determined from the first compressor speed or the turbine speed) is below the threshold speed, then at 217 the method includes maintaining operation of the first compressor (of the turbocharger) by maintaining the position of the one or more actuators at the current position. In particular, the method comprises continuing to flow compressed air to the piston engine via the (only) first compressor, and while bypassing the second compressor by maintaining the bypass valve at the current position, while continuing to rotate the first compressor.

In response to the shaft speed of the turbocharger being equal to or above the threshold rotational speed, the method includes accelerating the second compressor to reduce the load (and thereby reduce the shaft speed) of the first compressor. Specifically, at 218, the method includes estimating a second compressor speed (or supercharger power) required to reduce the shaft speed of the first compressor based on the shaft speed relative to the threshold while maintaining boost pressure of the engine.

Wherein first, a desired boost pressure may be determined based on a desired mass airflow (driver torque demand). The shaft speed of the turbocharger may then be calculated (i.e., predicted) based on engine operating conditions.

If the turbocharger shaft speed (or the predicted turbine speed or the first compressor speed), as predicted based on the desired boost pressure, is greater than the maximum speed limit, then the supercharger power (specifically, supercharger speed) required to reduce the turbocharger shaft speed to a safe value may be determined using a model based on the desired pressure ratio and the estimated air mass flow.

At 220, the method includes comparing the estimated second compressor speed to a threshold speed. The threshold speed may be based on the maximum power available from the electro-mechanical supercharger. If the estimated second compressor speed is below the threshold speed of the second compressor (i.e., the power requested from the supercharger is less than the maximum power that can be drawn from the supercharger), then at 222 the method includes accelerating the second compressor to reduce the load and the shaft speed of the first compressor. Accelerating the second compressor includes not bypassing the second compressor while continuing to provide compressed air via the first compressor. The accelerating further includes operating the electric motor at a speed based on the outlet temperature of the first compressor relative to a threshold, the speed of the electric motor increasing when the outlet temperature of the first compressor exceeds the threshold. Further, the electric motor may be accelerated by the controller signaling the actuator that closes or reduces slippage of a clutch coupled between the engine crankshaft and the second compressor, thereby transferring engine torque from the crankshaft to the second compressor. The second compressor may be accelerated until the turbocharger shaft speed is below the threshold speed. The second compressor may then be slowed down and boost pressure may resume delivery via only the first compressor.

If the estimated rotational speed of the second compressor required for controlling the turbocharger shaft speed is above the threshold rotational speed of the second compressor (i.e., the power requested from the supercharger is equal to or greater than the maximum power that can be drawn from the supercharger), then at 224 the method includes adjusting the ratio of power delivered from the electric motor to the second compressor relative to the crankshaft (as detailed in FIG. 3). Likewise, the supercharger compressor may be mechanically driven via a crankshaft coupled to the engine and/or electrically driven via an electric motor. In one example, in response to the turbocharger shaft speed exceeding a threshold, the drive of the second compressor via the crankshaft may be increased while the drive of the second compressor via the electric motor may be correspondingly decreased. By adjusting the ratio of driving the second compressor via the crankshaft relative to via the electric motor based on the outlet temperature of the first compressor, the supercharger compressor speed may be used to control the turbocharger speed without causing boost errors or overheating the supercharger compressor.

At 226, the method further includes accelerating the second compressor while limiting engine torque. Limiting engine torque may include one or more of reducing boost pressure and reducing engine intake airflow through the first compressor. For example, intake airflow may be reduced by adjusting an electromechanical actuator coupled to a throttle plate in the intake system to rotate the throttle valve to a smaller open position by sending a control signal from the controller to the actuator. For example, engine torque output may be subtracted and boost pressure may be correspondingly decreased. The boost pressure may be reduced by the controller sending a signal to a combination of an actuator, including but not limited to a wastegate actuator, the first compressor, and the CRV. For example, the signal may increase the rotation of the first compressor, decrease the opening of the wastegate, decrease the opening of the CRV, or a combination thereof. Here, the limit may be based on a difference between the estimated compressor speed and a threshold speed. For example, when the second compressor speed (or power) exceeds a threshold speed, the limit may be increased and the amount of intake airflow delivered to the engine may be decreased.

Turning now to fig. 3, a method 300 depicts a routine for adjusting the ratio of power delivered to a supercharger compressor via an electric motor relative to an engine crankshaft.

At 302, the method includes confirming that a second compressor acceleration (of the supercharger compressor) has been requested. In one example, a second compressor acceleration may have been requested to reduce the load and thus the shaft speed of the upstream turbocharger compressor. If no second compressor acceleration is requested, then at 304, the supercharger compressor may remain disabled and compressed air may flow to the engine via the first compressor bypassing the second compressor.

If acceleration of the second compressor is requested, then at 306, the method includes adjusting a ratio of mechanically driving the second (supercharger) compressor via the crankshaft relative to electrically driving the second (supercharger) compressor via the electric motor. The adjustment may be based on engine operating parameters such as first compressor shaft speed, turbine speed, first compressor temperature, battery state of charge, and the like. At 307a, the adjustment may include increasing mechanical power delivered from the engine crankshaft to the supercharger compressor while correspondingly decreasing output from the electric motor. In another example, the adjustment may include increasing the output from the electric motor while correspondingly decreasing the mechanical power from the engine crankshaft to the supercharger compressor (e.g., by increasing the clutch between the sliding crankshaft and the supercharger compressor shaft by sending a signal from the controller to the clutch actuator). As one example, a relatively high ratio of mechanically driving the secondary (supercharger) compressor via the crankshaft relative to electrically driving the secondary (supercharger) compressor via the electric motor may be applied when the battery state of charge is low. As another example, a relatively high ratio of mechanically driving the second (supercharger) compressor via the crankshaft relative to electrically driving the second (supercharger) compressor via the electric motor may be imposed when the first compressor outlet temperature is high.

At 308, it may be determined whether the turbocharger shaft speed or the first compressor speed is below a threshold speed. If not, the method includes maintaining the second compressor on and accelerating the second compressor until the first compressor shaft speed is below the threshold speed. At 310, the second compressor may be decelerated and disabled when the first compressor shaft speed is below the threshold rotational speed. Thereafter, charge air may be provided to the engine from only the first compressor.

In this manner, a method for supercharging an engine includes, during a first condition, in response to a shaft speed of a first upstream compressor being above a threshold, accelerating a second downstream compressor at a first rotational speed while maintaining boost pressure; and during a second condition, in response to the shaft speed of the first compressor being above the threshold, accelerating the second compressor at a second rotational speed while decreasing the boost pressure. Here, the second rotational speed is higher than the first rotational speed. In the above embodiment, during each of the first condition and the second condition, compressed air flows to the piston engine via the first compressor, and accelerating the second compressor includes stopping bypassing the second compressor. In any of the foregoing embodiments, the difference between the shaft speed and the threshold value during the second condition is greater than during the first condition. Further, the second compressor is driven by one or more of a battery operated electric motor and an engine crankshaft, while the first compressor is driven by the turbine. Further, during the second condition, acceleration of the second compressor is performed with a lower ratio of motor torque from the electric motor relative to engine torque from the crankshaft, while during the first condition, acceleration of the second compressor is performed with a higher ratio of motor torque from the electric motor relative to engine torque from the crankshaft. In any of the foregoing embodiments, each of the first and second conditions includes an increase in driver demand, the increase in driver demand during the second condition being greater than the increase in driver demand during the first condition.

Turning now to fig. 4, an example graph 400 illustrates a method for improving shaft speed control for an upstream turbocharger via operation of a downstream supercharger. Graph 400 depicts Pedal Position (PP) at curve 402, intake throttle opening at curve 404, boost pressure at curve 406, rotational speed of the first upstream compressor of the turbocharger (rev _ compressor 1) at curve 408, rotational speed of the turbine coupled to the first compressor via the turbocharger shaft (rev _ turbo) at curve 410, rotational speed of the second downstream compressor of the supercharger (rev _ compressor 2) at curve 414, and state of charge of the system battery coupled to the electric motor of the supercharger at curve 418. All curves are shown in time along the x-axis. Note that elements aligned at the same time on the chart (e.g., such as at time t1) occur simultaneously, including, for example, where one parameter increases while the other decreases.

Prior to t1, the engine may be operating unpressurized due to the lower operator torque request and vehicle speed. At t1, boost may be requested in response to tip-in (curve 402). To reduce turbo lag, the second compressor is accelerated for a duration of t1 to t2 to increase boost pressure in response to a boost demand. Here, the second compressor may be accelerated via operation of the electric motor, resulting in a corresponding drop in battery state of charge.

Between t1 and t2, as supercharged engine operation ensues, and exhaust gas temperature and pressure increase, the turbine of the turbocharger may accelerate (boost up), enabling the first compressor to accelerate at t 2. At t2, once the first compressor is accelerated and running, the second compressor may be decelerated. Additionally, air compressed by the first compressor may be delivered to the engine bypassing the second compressor. Between t2 and t3, the torque request and boost pressure may be provided via the first compressor only.

At t3, the turbocharger compressor may reach the threshold speed 412. The threshold speed may correspond to a hardware limit, continuing with the above-described operation, which can adversely affect a turbocharger component (such as a turbocharger shaft coupling the first compressor to the turbine). Likewise, due to the coupling, the turbocharger turbine may also reach the threshold speed 412 at t 3. Thus, to enable shaft speed control, at t3, the supercharger is reactivated and the second compressor is accelerated for the duration of t3 to t 5. Specifically, the second compressor is operated at a rotational speed that allows the load of the first compressor to be reduced while maintaining the boost pressure. The reduction in the first compressor load allows the rotational speed of the first compressor to be reduced, and thus the first turbine speed and the turbocharger shaft speed to be reduced. Likewise, in the absence of second compressor operation, the first compressor speed may continue to increase, as indicated by dashed line 409, and the turbine speed may also continue to increase, as indicated by dashed line 411.

In particular, it is determined that the second compressor speed corresponds to a supercharger power or load that allows turbocharger speed control. In the depicted example, the second compressor speed (curve 414) is within the limits 416 of the supercharger. In an alternative example, if the second compressor speed required to control the turbocharger overspeed condition is above limit 416, the supercharger compressor may be operated at the speed limit (as indicated by dashed segment 415) while limiting boost pressure (as indicated at dashed segment 407). In one example, boost pressure is limited by reducing intake airflow by reducing the opening of the intake throttle (as indicated at dashed segment 405). Limiting boost pressure and supercharger compressor acceleration may enable turbocharger speed to be controlled.

In one example, between t3 and t5, the supercharger compressor may be accelerated via power from the electric motor, with a resultant drop in battery state of charge. However, in the depicted example, since the battery state of charge becomes low at t4, between t4 and t5, supercharger compressor acceleration may be achieved by reducing the electric motor output (or motor torque) delivered to the supercharger compressor (as indicated by no further drop in battery SOC), and by increasing the engine torque delivered to the supercharger compressor (via the engine crankshaft).

At t4, when the first compressor speed is sufficiently under control, the second compressor of the supercharger may be decelerated and the first compressor of the turbocharger may be accelerated to meet the boost requirement.

In one example, an engine system includes: an engine having an intake air; a first intake compressor driven by the exhaust turbine; a second intake compressor driven by an electric motor, the motor being powered by the battery, the second compressor being positioned downstream of the first compressor along the intake air; a rotational speed sensor coupled to a shaft of the first compressor; and a controller. The controller may be configured with computer readable instructions stored on non-transitory memory for: operating the second compressor while disabling the first compressor until the turbine speed is above a threshold turbine speed in response to the accelerator pedal being stepped on; then, bypassing the second compressor while continuing to operate the first compressor; and rotating the second compressor in response to the shaft speed of the first compressor being above the threshold rotational speed while continuing to operate the first compressor. In the foregoing embodiment, the controller may include further instructions for rotating the second compressor until the shaft speed of the first compressor is below the threshold rotational speed, and after the shaft speed of the first compressor is reduced, disabling the second compressor and providing compressed air to the engine via only the first compressor. In the foregoing embodiment, rotating the second compressor includes rotating the electric motor at a speed based on a difference between the shaft speed and a threshold speed, the electric motor speed increasing to the threshold motor speed as the difference increases, maintaining boost pressure of the engine while the electric motor speed increases to the threshold motor speed. In any of the preceding embodiments, the controller comprises further instructions for: after the electric motor speed is increased to the threshold motor speed, the electric motor speed is maintained at the limit while limiting the boost pressure, the limiting based on the shaft speed. In any of the foregoing embodiments, limiting boost pressure includes reducing a speed of the first compressor and/or reducing an opening of an intake throttle valve positioned upstream of the first compressor in engine intake air.

In another representation, a method for supercharging an engine includes providing a flow of compressed air to a piston engine bypassing a first compressor via a second compressor, the second compressor positioned downstream of the first compressor, the first compressor coupled to a turbine. In response to the turbine speed being above the first threshold speed, the second compressor is decelerated and a flow of compressed air is provided to the piston engine via the first compressor, bypassing the second compressor. Then, in response to the turbine speed being higher than a second threshold speed, which is higher than the first threshold speed, the second compressor is accelerated and a flow of compressed air is provided to the piston engine via each of the first and second compressors. Herein, the first threshold is based on turbo lag, and the second threshold is based on hardware limits.

In another expression, a method for supercharging an engine includes, during tip-in, providing compressed air to the engine bypassing a first upstream compressor via a second downstream compressor until a first compressor speed reaches a first threshold. Here, the first compressor is driven by the turbine, while the second compressor is at least partially driven by the electric motor. Providing compressed air via the second compressor includes accelerating the second compressor via the electric motor. Then, a transition is made to providing compressed air to the engine via the first compressor bypassing the second compressor until the first compressor speed reaches a second threshold value that is higher than the first threshold value. Bypassing the second compressor includes opening a bypass valve and/or decelerating the second compressor in a bypass coupled across the second compressor. In response to the first compressor speed reaching the second threshold, the method further includes accelerating the second compressor until the first compressor speed is below the second threshold but above the first threshold. Compressed air is provided to the engine via each of the first and second compressors until the first compressor speed is controlled.

In this way, the electro-mechanical supercharger may be used to reduce the likelihood of overspeed at an upstream turbocharger (such as in an aggressively miniaturized engine). By using a supercharger to reduce the load and speed of the turbocharger compressor, hardware constraints can be reduced without incurring a loss in boost pressure and drivability. In general, turbocharger component life is consumed without degrading the engine's ability to meet driver demand torque.

It should be noted that the example control and estimation routines included herein may be used with various engine and/or vehicle system configurations. The control methods and programs disclosed herein may be stored as executable instructions in non-transitory memory and may be implemented by a control system including a controller in combination with various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts, operations, and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts, operations, and/or functions may graphically represent code to be programmed into the non-transitory memory of the computer readable storage medium in an engine control system, wherein the described acts are accomplished by executing instructions in a system that includes various engine hardware components in combination with an electronic controller.

It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above-described techniques may be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to "an" element or "a first" element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.

Claims

1. A method for supercharging an engine, the method comprising:

bypassing a second compressor and providing a flow of compressed air to a piston engine via a first compressor, wherein the first compressor is staged downstream of the second compressor along an air intake; and

accelerating the second compressor in response to a shaft speed of the first compressor being equal to or above a threshold.

2. The method of claim 1, further comprising accelerating the second compressor until the shaft speed of the first compressor is below the threshold and then decelerating the second compressor.

3. The method of claim 1, wherein the threshold is a first threshold, wherein accelerating the second compressor comprises estimating a second compressor speed required to reduce the shaft speed of the first compressor based on the shaft speed of the first compressor relative to the first threshold, and accelerating the second compressor to a second threshold when the estimated second compressor speed is below the second threshold, and adjusting a ratio of power delivered from an electric motor to a second compressor relative to an engine crankshaft while limiting engine torque when the estimated second compressor speed is above the second threshold.

4. The method of claim 3, wherein limiting engine torque comprises limiting engine torque based on a difference between the estimated second compressor speed and the second threshold.

5. The method of claim 3, wherein limiting engine torque comprises one or more of reducing boost pressure and intake airflow through the first compressor.

6. The method of claim 1, wherein accelerating the second compressor comprises not bypassing the second compressor while continuing to provide compressed air via the first compressor.

7. The method of claim 3, wherein the second threshold is determined based on a maximum power available from an electric supercharger.

8. The method of claim 1, wherein the second compressor is driven by an electric motor, and wherein the first compressor is driven by an exhaust turbine via a shaft.

9. The method of claim 8, wherein accelerating the second compressor includes operating the electric motor at a rotational speed based on the shaft speed relative to the threshold, the rotational speed of the electric motor increasing when the shaft speed of the first compressor exceeds the threshold.

10. A method for supercharging an engine, the method comprising:

during a first condition, in response to the shaft speed of the first downstream compressor being above a threshold, accelerating the second upstream compressor at a first rotational speed while maintaining boost pressure; and

during a second condition, in response to the shaft speed of the first downstream compressor being above the threshold, the second upstream compressor is accelerated at a second rotational speed while decreasing boost pressure.

11. The method of claim 10, wherein the second rotational speed is higher than the first rotational speed.

12. The method of claim 10, wherein during each of the first and second conditions, compressed air flows to a piston engine via the first downstream compressor, and wherein accelerating the second upstream compressor comprises ceasing to bypass the second upstream compressor.

13. The method of claim 10, wherein the first downstream compressor is driven by a turbine through a shaft and the second upstream compressor is driven by one or more of a battery-operated electric motor and an engine crankshaft, and wherein a difference between the shaft speed and the threshold is greater during the second condition as compared to the first condition.

14. The method of claim 13, wherein during the second condition, the accelerating of the second upstream compressor is performed with a lower ratio of motor torque from the electric motor relative to engine torque from the crankshaft, and during the first condition, the accelerating of the second upstream compressor is performed with a higher ratio of motor torque from the electric motor relative to engine torque from the crankshaft.

15. The method of claim 10, wherein each of the first and second conditions includes an increase in driver demand, and wherein the increase in driver demand during the second condition is greater than the increase in driver demand during the first condition.

16. An engine system, the engine system comprising:

an engine having an intake air;

a first intake compressor driven by the exhaust turbine;

a second intake compressor driven by an electric motor, the motor being powered by a battery, the second intake compressor being positioned along the intake air upstream of the first intake compressor;

a rotational speed sensor coupled to a shaft of the first intake compressor; and

a controller having computer readable instructions stored on non-transitory memory for:

in response to the tip-in of the accelerator pedal,

operating the second intake compressor while disabling the first intake compressor until a turbine speed is above a threshold turbine speed;

then bypassing the second intake compressor while operating the first intake compressor; and

rotating the second intake compressor in response to the shaft speed of the first intake compressor being above a threshold shaft speed while continuing to operate the first intake compressor.

17. The system of claim 16, wherein the controller comprises further instructions to: rotating the second intake compressor until the shaft speed of the first intake compressor is below the threshold shaft speed, and after the shaft speed of the first intake compressor is reduced, disabling the second intake compressor and providing compressed air to the engine only via the first intake compressor.

18. The system of claim 16, wherein rotating the second intake compressor includes rotating the electric motor at a speed based on a difference between the shaft speed and the threshold shaft speed, the electric motor speed increasing to a threshold motor speed as the difference increases, a boost pressure of the engine being maintained while the electric motor speed increases to the threshold motor speed.

19. The system of claim 18, wherein the controller comprises further instructions to: after the electric motor speed is increased to the threshold motor speed, maintaining the electric motor speed at the threshold motor speed while limiting boost pressure, the limiting based on the shaft speed.

20. The system of claim 19, wherein limiting boost pressure comprises reducing a speed of the first intake compressor and/or reducing an opening of an intake throttle positioned upstream of the first intake compressor in the engine intake.